In general, a microphone is a device converting sound into an electrical signal. Microphones may be used to various applications such as mobile communication devices like smartphones, earphones or hearing aids.
The microphones have been increasingly downsizing in recent years, and accordingly, micro electro mechanical system (MEMS) microphones employing the MEMS technology have been developed.
The MEMS microphones are manufactured through a semiconductor batch process. They are more resistant to moisture and heat than the conventional ECMs (Electret Condenser Microphones) and are well-suited for downsizing and easily integrated into a signal processing circuit.
These MEMS microphones are classified into a piezoelectric type and a capacitive type.
The piezoelectric type MEMS microphone consists of a vibration membrane only. When a vibration membrane is deformed by external sound pressure, an electrical signal is generated due to a piezoelectric effect, by which the sound pressure is measured.
The capacitive type MEMS microphone includes a vibration membrane and a fixed membrane. When the vibration membrane is subject to an inflow of external sound pressure, capacitance between the vibration and fixed membranes changes as the gap between them is varied due to vibration of the vibration membrane. Here, the varying capacitance value is output as a voltage signal and is expressed as sensitivity, which is one of important performance indices.
The current MEMS microphones under development are unchangeable due to the fixed gap between the vibration and fixed membranes. The gap between the vibration and the fixed membrane may change according to residual stress of the vibration or the fixed membrane and the thickness of a sacrificial layer deposited between the membranes.
The gap between the vibration and the fixed membrane exerts a large influence over the sensitivity and the noise, which are the most important performance indices of the MEMS microphone. In this regard, research and development for ensuring reproducibility is most needed.
The specifics in this background section are intended to enhance understanding of the background of the invention and may include those specifics not belonging to the conventional art already known to those skilled in the art to which the present disclosure belongs.